The problem of collision avoidance is often present in the case of operation of elevators with at least two cages movable along a common travel path.
A safety device is proposed in European Patent Specification 1 562 848 A1, which takes account of the above-mentioned problem. This safety device prevents a collision between two cages in that the safety device monitors whether the cages maintain a critical safety spacing. If this critical safety spacing is fallen below, the safety device initiates an emergency stop. The safety device additionally monitors the spacing between the two cages during execution of the emergency stop. If notwithstanding the emergency stop a further approach of the cages takes place and in that case a minimum safety spacing is fallen below, then the safety device initiates safety braking.
The above safety device was further refined in European Patent Specification 1 698 580 A1. Here, too, the safety device continuously monitors a critical safety spacing and in a given case a minimum safety spacing and if the respective safety spacing is fallen below appropriately initiates an emergency stop or a safety braking. These safety spacings are, however, determinable on the basis of a predeterminable emergency stop trigger plot and a predeterminable safety brake trigger plot. This can mean that a respective speed-dependent critical or minimum safety spacing is determinable for the instantaneous travel speed of a cage. Correspondingly, the cages can in the case of a lower travel speed approach to a further extent without a braking measure being initiated. This makes possible, in particular, approach of the cages to two adjacent stories.
However, in the case of the two above-mentioned two-stage braking procedures the spacing of the two elevator cages is usually continuously monitored and compared with a critical and a minimum safety spacing. This continuous monitoring of the spacing can impose relatively high demands on the computing capacity of the safety device. This applies particularly in the case of calculation, in dependence on trigger plot, of the safety spacings of the two braking procedures.